Rechargeable batteries for use as power sources in portable electronic equipment are required to have a high energy density and also required to have space efficient configuration in keeping with the demand for weight reduction and miniaturization in the electronic equipment. As a battery that satisfies these requirements, increasing attention has been paid to a prismatic lithium rechargeable battery employing an aluminum-made, relatively flat battery case in the form of a rectangular prism. This lithium rechargeable battery has non-aqueous electrolyte (organic solvent-base electrolyte) contained in the battery case and is thus required to exhibit stable hermeticity for a longer period of time. Thus, in manufacturing the lithium rechargeable battery, after an electrode group is housed in the battery case in the form of a rectangular prism with a bottom, an opening of the battery case is sealed with a sealing plate by laser welding.
In a non-aqueous electrolyte rechargeable battery, like the lithium rechargeable battery described above, when it is overcharged, or a short circuit occurs due to wrong use and consequently the non-aqueous electrolyte is decomposed, gas is generated. The resultant gas fills inside the battery case hermetically sealed, and, if this causes the internal pressure of the battery to rise to a certain value or above, the battery case may possibly be ruptured. Non-aqueous electrolyte rechargeable batteries in particular are more susceptible to such a failure than batteries of other type.
In order to protect a battery case from rupture, there has conventionally been provided a safety mechanism whereby, when an internal pressure of the battery case exceeds a certain value, an opening is created in a part of the battery case by exploiting the pressure, and, through the resultant opening, the gas is discharged out of the battery case.
Examples of generally-known safety mechanisms for use with non-aqueous electrolyte rechargeable batteries are shown in FIGS. 11A to 11C. A first conventional safety mechanism shown in FIG. 11A is constructed as follows. In a battery case 1 in the form of a rectangular prism with a bottom, an aluminum-made sealing plate (which is 900 μm in thickness, for example) 2 is provided for sealing an upper-end opening of the battery case 1. The sealing plate 2 has a gas vent hole 3 formed in a part thereof. Moreover, an aluminum-made thin plate (which is 30 μm in thickness, for example) 4 is bonded to an under surface of the sealing plate 2 in a vacuum. In this construction, when an internal pressure of the battery case 1 rises to a certain value or above, a portion 4a of the thin plate 4 for covering the gas vent hole 3 is pressurized by the gas pressure and is thereby ruptured. As a result of the rupture of the thin plate 4, an opening is created, and, through the resultant opening and the gas vent hole 3, the internal gas is discharged out of the battery case 1.
A second conventional safety mechanism shown in FIG. 11B is constructed as follows. In a battery case 1 in the form of a rectangular prism with a bottom, a marking groove 7 is formed on one of elongated side surfaces 1a thereof. The marking groove 7 has a V-shaped section and appears circular when viewed from the plane. Between a groove bottom surface of the marking groove 7 and an inner surface of the battery case 1 is disposed a thin-walled and circular easily-rupturable portion 8. In this construction, when the internal pressure of the battery case 1 rises to a certain value or above, the easily-rupturable portion 8, which is lower in strength than the other portions of the elongated side surface 1a, is cleaved and opened. Through the resultant opening, the gas is discharged to the outside.
A third conventional safety mechanism shown in FIG. 11C is constructed as follows. On a bottom surface of a battery case 1 is formed a marking groove 9 which is composed of a linear portion parallel to a ridge line of the battery case 1, and V-shaped portions extending from both ends of the linear portion. In this construction, the marking groove 9 is formed on the bottom surface 1b, i.e. the minimum-area portion, of the battery case 1. Thus, when both of the elongated side surfaces 1a and 1c of the battery case 1 are expanded outwardly with a rise in the internal pressure, the bottom surface 1b is deformed inwardly, and whereby the marking groove 9 is cleaved and opened. Through the resultant opening, the gas is discharged to the outside.
However, the first safety mechanism requires a drilling process for forming the gas vent hole 3 in the relatively small sealing plate 2 having a rectangular shape, a process for activating one surface of the sealing plate 2 that becomes an inner surface when mounted in the battery case 1, and a vacuum adsorption process for combining the thin plate 4 with the activated surface by pressing using a roller. This leads to an undesirable increase in the manufacturing cost. Furthermore, since the gas vent hole 3 is formed in an end portion of the sealing plate 2 having a relatively small configuration, the action of the gas pressure required to rupture the portion 4a or covering the gas vent hole 3 is exerted only on a strictly localized region of the battery case 1 as a whole. This necessitates much time being spent in rupturing the portion 4a for covering the gas vent hole 3. To address this problem, conventionally, the thickness of the thin plate 4 has been reduced from 30 μm to 20 μm in an attempt to rupture the thin plate 4 rapidly at the instant when the battery internal pressure reaches a predetermined value. However, reducing the thickness of the thin plate 4 creates another problem that the thin plate 4 may possibly be ruptured by an impact caused by drop tests.
Moreover, in the second safety mechanism, the elongated side surface 1a, which has a thickness of approximately 300 μm, of the battery case 1 is subjected to press working using a punching die or the like to form the circular, wedge-like marking groove 7. Thereby, a remaining wall thickness of approximately 80 μm is obtained to form the easily-rupturable portion 8. In this case, thermal stress developed during the press working causes work hardening, with the result that, in the elongated side surface 1a, the physical properties of the periphery of the easily-rupturable portion 8 are varied. Since the degree of the change in the physical properties, that is, the degree of hardening and embrittling is not kept constant, it is impossible to set the battery internal pressure for the rupture of the easily-rupturable portion 8 at a fixed value. In addition, to prevent accidental intrusion of dust or dirt into the battery case 1, the marking groove 7 is formed in the battery case 1 with its opening temporarily sealed by the sealing plate. At this time, the sealing plate 2 is subjected to stress resulting from the flow of the material during the press working is conducted on the elongated side surface 1a having a relatively large area, and is thereby slightly opened with respect to the opening of the battery case 1. Thus, when the sealing plate 2 is fixed to the battery case 1 by laser welding, a blowhole is created. The resultant small hole tends to cause leakage of electrolyte.
Further, in the third safety mechanism, the marking groove 9 is formed on the bottom surface 1b, i.e. the minimum-area portion of the battery case 1. Thus, unlike the second safety mechanism, the third safety mechanism is free of such a problem that the sealing plate 2 is inconveniently opened during press working. However, since the marking groove 9 is formed by press working, as observed in the second safety mechanism, work hardening occurs during the press working and this causes the physical properties of the periphery of the marking groove 9 to change. Consequently, the battery internal pressure cannot be kept constant for the operation of the safety mechanism. In addition, the marking groove 9 is formed on the bottom surface 1b, i.e. the minimum-area portion of the battery case 1, which is resistant to deformation under a rising internal pressure. Thus, in order for the safety mechanism to operate at a predetermined battery internal pressure, the remaining wall thickness of the marking groove 9 needs to be kept as small as possible. The wall thickness needs to be controlled with high accuracy, and this leads to poor workability of the marking groove 9. What is worse, the remaining wall thickness of the groove bottom portion of the marking groove 9 is so small that the resistance to falling is extremely low. Further, another problem arises. In a case where the battery pack is constituted by placing a combination of a plurality of cells connected in series or parallel with one another in a pack case, formation of the marking groove 9 on the bottom surface 1b of the battery case 1 makes difficult connection of leads by welding.
The present invention has been made in light of the above stated problems with the conventional art, and accordingly it is an object of the present invention to provide a prismatic battery safety mechanism that, despite being constructed at lower cost, discharges gas out of a battery case properly and swiftly at an instant when a battery internal pressure reaches a predetermined value, and it is also an object of the invention to provide a method for easily manufacturing the safety mechanism while preventing occurrence of problems.